The invention concerns a rotary mechanism of the trochoidal type such as shown in U.S. Pat. No. 2,988,065 granted June 13, 1961 to Wankel et al. Such a rotary mechanism comprises an outer body or housing having two end housing parts and an intermediate or rotor housing part within which a rotor is mounted on the eccentric portion of a shaft extending coaxially through the housing. The inner surface of the rotor housing of such a rotary mechanism is substantially an epitrochoid. Also, the peripheral surface of the rotor approximates the inner envelope of this trochoidal surface, sufficient clearance being provided between the rotor periphery and the trochoid surface to avoid mechanical interference therebetween. As disclosed in said prior patent, the epitrochoid surface may have two lobes in which case the rotor has three apex portions and rotates at one-third the speed of the eccentric shaft.
Such a rotary mechanism may, for example, be operated as a rotary combustion engine as well as a compressor or expansion motor. The shape of the epitrochoid surface determines the maximum compression ratio of such a rotary mechanism. This shape of the epitrochoid is determined by the ratio R to e where R is the epitrochoid generating radius, that is, the radial distance from the center of the rotor to the tip of its apex portions, and e is the distance between the axis of the rotor and the axis of the engine shaft. This ratio (R/e) is generally known as the "K" factor. Higher compression ratios can be obtained by increasing the K factor of the epitrochoid surface.
As disclosed in said prior patent, a rotary mechanism in which the epitrochoid has a relatively large K factor has several drawbacks. For example, with a larger K factor the engine size must be increased in order to provide an engine with working chambers of a given size. Also, as the K factor is increased, the diameter of the engine shaft must be decreased.
For the purpose of increasing the available compression ratio but without the aforementioned difficulties, it is known in the prior art to start with a trochoid having a relatively low K factor and then for the purpose of increasing the compression ratio the trochoid is modified by displacing it outwardly in the region of the trochoid minor axis and the periphery of the rotor is modified so that the rotor periphery substantially corresponds to the inner envelope of the modified trochoid. Such a prior art rotary mechanism is disclosed in German Pat. No. 1,158,317 granted Nov. 28, 1963 to Klockner-Humboldt-Deutz and German Pat. No. 1,164,746 granted Mar. 5, 1964 to Klockner-Humboldt-Deutz. In this latter German patent, each of said modified portions of the trochoid surface comprises a circular arc. In this way it is possible to increase the compression ratio without increasing the trochoid K factor.
With this modified trochoid construction of said German patents there is an abrupt change in the radial acceleration or jerk of the apex seals as the seals move over the beginning and end of the modified portions of the trochoid surface. This abrupt change in acceleration of the apex seals could cause vibration of these seals relative to rotor and trochoid surface. Also, such a modification of the trochoid surface results in considerable radial movement of the rotor apex seals as they move over the modified portions of the trochoid surface in order for these seals to maintain contact with said surface.
Theoretically, radial motion of the rotor apex seals does not occur if the trochoid surface iss made in the form of a parallel curve spaced radially outwardly from a true epitrochoid a distance equal to the radius of the tip of each apex seal of the rotor. Such a parallel curve trochoid construction is disclosed in U.S. Pat. No. 2,988,008 granted June 13, 1961 to Wankel.